Method and system for optimizing the performance of a rotodynamic multi-phase flow booster

ABSTRACT

A method and system for optimizing the performance of a rotodynamic multi-phase flow booster, such as a wet gas compressor ( 1 ), utilizes a meter which measures the fluid density ρ upstream of the booster and controls the speed of rotation Ω of the rotor(s) of the booster, such that the booster operates at its best efficiency point (BEP) irrespective of variations of the fluid density ρ. Suitably the flow booster is a wet gas compressor ( 1 ) having rotor and/or stator blades having a rough leading edge and non-wettable sides which promote a favorable finely dispersed mist flow so that the compressor ( 1 ) also operates at its BEP if the compressed wet gas has a high liquid content.

BACKGROUND OF THE INVENTION

[0001] The invention relates to a method and system for optimizing theperformance of a rotodynamic multi-phase flow booster, such as a rotarypump or compressor for pumping a multi-phase gas/liquid mixture througha fluid transportation conduit.

[0002] Various types of rotodynamic pumps or compressors are known forboosting multi-phase flow. International patent application WO93/04288discloses a contra-rotating pump or compressor of which the impellerblades are mounted on contra-rotating sleeves.

[0003] UK patent 2215408 discloses a system for controlling thegas-liquid ratio in a pump, wherein the gas-liquid ratio is maintainedat a substantially constant level by extracting liquid from thedownstream end of the pump and feeding the extracted liquid back to thepump inlet if the measured gas/liquid ratio exceeds a predeterminedlevel.

[0004] U.S. Pat. No. 5,472,319 discloses an eccentric twin screw pumpwith liquid bypass controlled by a flexible diaphragm.

[0005] A disadvantage of the known multi-phase boosters is that thepumping efficiency varies in response to the density of the fluidmixture passing through the booster.

[0006] This variation can be particularly high in case gas and liquidslugs are passing alternatingly through the booster.

[0007] U.S. Pat. No. 3,568,771 discloses an electrical submersible oilwell pump for lifting foamy crudes in which the rotary speed of the pumprotor(s) is varied as a function of the bulk density of the crude in thewell bore. European patent application No. 0549439 discloses amulti-phase pump of which the speed of rotation is varied in response tovariation of the gas/liquid ratio of the pumped multi-phase mixture.

[0008] An object of the present invention is to provide a method andsystem for further optimizing the performance of a rotodynamic flowbooster, such as a gas/liquid compressor, such that the pumpingefficiency is maintained at its best efficiency point if the density ofthe fluid mixture passing through the pump varies.

SUMMARY OF THE INVENTION

[0009] The method according to the invention for optimizing theperformance of a rotodynamic multi-phase flow booster comprisesmeasuring the density of the multi-phase fluid flow; and controlling thespeed of rotation of a rotor of the pressure booster, characterized inthat the density of the multi-phase fluid flow is measured at a locationupstream of the rotary pressure booster, and in that the non-dimensionalrotational speed Ω of said rotor is controlled in relation to themeasured non-dimensional density ρ of the multi-phase fluid on the basisof the algorithm:

Ω=f·ρ ^(n)

[0010] wherein:

[0011] n is an exponent between −⅓ and −1 which is selected in relationto the mode of operation; and

[0012] f is a parameter which is determined by the geometry of thebooster and the mode of operation, wherein the mode of operation isselected such that if the fluid mixture density ρ varies either a) themixture mass flow passing through the booster is maintained at asubstantially constant level; or b) the power consumption of the boosteris maintained at a substantially constant level; or c) the pressuredifference between the outlet and the inlet of the booster is maintainedat a substantially constant level.

[0013] Under the foregoing conditions the booster is expected tocontinue to operate at its best efficiency point (BEP) as with varyingfluid mixture density and compositions the impellers or screws of therotor(s) of the booster continuously create favourable fluid flowconditions such as a dispersed mist flow in which liquid droplets arefinely dispersed in the gas phase or a dispersed bubble flow in whichgas bubbles are finely dispersed in the liquid phase.

[0014] Suitably, the density and/or gas/liquid mass fraction of themulti-phase fluid mixture is measured by a gradio-venturi flowmeter or awet gas tracer apparatus which is located in the conduit upstream of theflow booster.

[0015] The system according to the invention for optimizing theperformance of a rotodynamic multi-phase flow booster comprises:

[0016] a density measuring device for measuring the density of themulti-phase flow; and

[0017] a rotary speed control unit for controlling the speed of rotationof a rotor of the booster in accordance with a predetermined algorithm,which controls the dimensionless speed of rotation Ω of said rotor,characterized in that the density measuring device is located upstreamof the flow booster, and in that the dimensionless speed of rotation Ωis controlled in relation to the measured non-dimensional density ρ ofthe multi-phase fluid on the basis of the algorithm:

Ω=f·ρ ^(n)

[0018] wherein:

[0019] n is an exponent between −⅓ and −1 which is selected in relationto the mode of operation; and

[0020] f is a parameter which is determined by the geometry of thebooster and the mode of operation, wherein the mode of operation isselected such that if the fluid mixture density ρ varies either a) themixture mass flow passing through the booster is maintained at asubstantially constant level; or b) the power consumption of the boosteris maintained at a substantially constant level; or c) the pressuredifference between the outlet and the inlet of the booster is maintainedat a substantially constant level.

[0021] The invention also relates to a rotodynamic multi-phase flowbooster for use as a wet gas compressor and which is equipped with oneor more rotor and/or stator parts having rotor and/or stator bladeswhich are designed to transform liquid droplets which in use impingeupon the leading edges of the blades into a mist of smaller dropletswhich are and remain a finely dispersed mist in the gaseous phase.

[0022] Furthermore the invention relates to a rotor and/or stator bladesuitable for use in such a wet compressor.

[0023] In such case the rotor blade comprises non-wettable sides, whichare suitably coated with a polytetrafluorethylene or PTFE (sold by E. I.du Pont de Nemours and Company under the trademark “TEFLON”) and aleading edge having a larger roughness than the sides.

[0024] Suitably the leading edge of the blade(s) is covered by a randomarray of grains having a grainsize of at least about 50 microns, whichgrains are separated by distances which are on average less than twicethe grainsize.

[0025] Alternatively the leading edge of the blade(s) is provided with aseries of riblets which have a square or triangular cross-sectionalshape, which are substantially aligned with the direction of flow of themulti-phase fluid mixture and which have mutual spacings of at least 50microns.

[0026] In a wet gas compressor which has a rotor and/or stator which isequipped with such rotor and/or stator blades a favourable mist flow ofsmall liquid droplets which are finely dispersed in the gaseous phasewill be maintained also if the wet gas has a high liquid content whichalso contributes to operation of the wet gas compressor at its bestefficiency point (BEP) in circumstances that the gas has a high liquidcontent, which may exceed 10% of the volume of the wet gas mixture.

DESCRIPTION OF A PREFERRED EMBODIMENT

[0027] The invention will be described in more detail with reference tothe accompanying drawing, in which:

[0028]FIG. 1 schematically shows a multi-phase flow booster, wherein therotary speed of the rotor(s) of the booster is varied in response tovariations of the density of the fluid mixture passing through thebooster; and

[0029]FIG. 2 shows a characteristic curve for operating the booster atits best efficiency point (BEP).

[0030] Referring to FIG. 1 there is shown a multi-phase fluid compressor1 having a rotor 2 of which the rotor speed Ω is controlled by anangular velocity control unit 3.

[0031] A fluid feed conduit 4 is connected to the inlet of thecompressor 1 and a fluid discharge conduit 5 is connected to the outletof the compressor 1.

[0032] A fluid density meter 6 is connected to the fluid feed pipe 4 andtransmits a signal which is representative of the measured fluid densityρ to the angular velocity control unit 3.

[0033] The fluid density meter 6 may be a gradio venturi flowmeter whichis described in U.S. Pat. Nos. 4,856,344; 5,361,206 or 3,909,603.Alternatively any other multi-phase flowmeter capable of measuring thefluid mixture density with cut-off frequency suitably chosen in-linewith booster characteristics. These could include wet gas tracer methodsfor spot measurement under stable conditions; however, for on-linedirect regulation a continuous measurement would be needed. Such acontinuous measurement may detect fluctuations of the mixture density,for instance in slugging conditions, enabling direct speed control.Continuous measurement would be possible using either two separate Δpflowmeters in series or one Δp flowmeter calibrated for the specificconditions.

[0034] The underlying operating principle for the multi-phase pump orcompressor according to the invention is the assumption of well-mixedflow (effectively a flow of ideally mixed gas and liquid phases). Fromfluid flow considerations the best efficiency point (BEP) is derived asa function of mixture density ρ and rotational speed Ω.

[0035] The algorithm for speed regulation can support different modes ofoperation, including: 1. Maintain constant pressure rise Δp (bar) 2.Maintain constant total mass flow rate Ωm (kg/s) 3. Maintain constantpower consumption (kW)

[0036] The optimal angular velocity control system of a pump orcompressor unit is expressed in these cases as:

Ω=f(ρ)^(n)  (1)

[0037] Here Ω and ρ are non-dimensional rotational speed and mixturedensity, respectively; exponent n is selected in the range between −⅓and −1, which depends on the mode of operation and f is a parameter thatis uniquely determined by the geometry of the pump or compressor unitand the operating conditions. Its value at the actual operatingconditions can be determined from regular performance data, for instanceby fitting the observed performance of the actual pump or compressor tothe performance predicted by the model.

[0038] In all three modes of operation, an increase of mixture densitymust be compensated by a decrease in angular velocity Ω of the rotor 2in order to remain at best efficiency point (BEP). To remain at bestefficiency point the characteristic curve shown in FIG. 2 should befollowed.

[0039] In the embodiment shown in FIG. 1 the rotor 2 is a propellerhaving two blades of which the sides are coated withpolytetrafluorethylene (PTFE) and of which the leading edges have alarger roughness than the sides, e.g. by covering the leading edges withan array of e.g. sandgrains like an array used in sandpaper.

[0040] In FIG. 2 the vertical axis represents the angular velocity Ω ofthe rotor and the horizontal axis the gas mass fractionm_(g)/(m_(l)+m_(g)).

[0041] The BEP curve shown in FIG. 2 is based on scaling theory whichestimates the performance of a given pump or compressor under operatingconditions. Because the BEP curve is derived from basic fluid mechanicsprinciples, booster performance may be predicted for operatingconditions within the parameter range for which experimental data isavailable. Outside this test range the theory allows to predict bestefficiency performance of the pump or compressor with reasonableaccuracy. In general compressor or pump performance is characterizedwith a number of dimensionless groups. For a wet gas compressor thesegroups can be characterized by assuming that the liquid phase issubstantially incompressible and that the gaseous phase is (weakly)compressible.

[0042] Application of the rules of dynamic scaling theory under thesemixed fluid flow considerations yields formula (1) and the BEP curveshown in FIG. 2.

1. A method for optimizing the performance of a rotodynamic multi-phaseflow booster (1), the method comprising measuring the density of themulti-phase fluid flow; and controlling the speed of rotation of a rotorof the pressure booster (1), characterized in that the density of themulti-phase fluid flow is measured at a location upstream of the rotarypressure booster (1), and in that the non-dimensional rotational speed Ωof said rotor is controlled in relation to the measured non-dimensionaldensity ρ of the multi-phase fluid on the basis of the algorithm: Ω=f·ρ^(n) wherein: n is an exponent between −⅓ and −1 which is selected inrelation to the mode of operation; and f is a parameter which isdetermined by the geometry of the booster (1) and the mode of operation,wherein the mode of operation is selected such that if the fluid mixturedensity ρ varies either a) the mixture mass flow passing through thebooster (1) is maintained at a substantially constant level; or b) thepower consumption of the booster (1) is maintained at a substantiallyconstant level; or c) the pressure difference between the outlet and theinlet of the booster (1) is maintained at a substantially constantlevel.
 2. The method of claim 1, wherein the density and/or gas/liquidmass fraction of the multi-phase fluid is measured by a gradio-venturiflowmeter (6) which is located in a conduit upstream of the pressurebooster (1).
 3. The method of claim 1, wherein the rotodynamic pressurebooster (1) has at least one rotor of which the speed of rotation isadjusted to a predetermined value by means of a rotary speed controlunit (3) which is programmed with the algorithm described in claim 1 andwhich controls the speed of rotation such that for each mixture densitythe booster (1) operates at its best efficiency point.
 4. The method ofclaim 3, wherein at the best efficiency point the impellers of the rotoror rotors of the booster (1) create and maintain favourable fluid flowconditions such as a dispersed mist flow in which liquid droplets areevenly dispersed in the gas phase.
 5. The method of claim 1, wherein theflow booster (1) is a wet gas compressor which is located downhole in agas production well.
 6. The method of claim 1, wherein the flow booster(1) is a wet gas compressor which is located above the earth surface. 7.The method of claim 6, wherein the flow booster (1) is a wet gascompressor which is located at the sea bottom or at a remote on-shorelocation.
 8. A system for optimizing the performance of a rotodynamicmulti-phase flow booster (1), the system comprising: a density measuringdevice (6) for measuring the density of the multi-phase flow; and arotary speed control unit for controlling the speed of rotation of arotor of the booster (1) in accordance with a predetermined algorithm,which controls the dimensionless speed of rotation Ω of said rotor,characterized in that the density measuring device (6) is locatedupstream of the flow booster (1), and in that the dimensionless speed ofrotation Ω is controlled in relation to the measured non-dimensionaldensity ρ of the multi-phase fluid on the basis of the algorithm: Ω=f·ρ^(n) wherein: n is an exponent between −⅓ and −1 which is selected inrelation to the mode of operation; and f is a parameter which isdetermined by the geometry of the booster (1) and the mode of operation,wherein the mode of operation is selected such that if the fluid mixturedensity ρ varies either a) the mixture mass flow passing through thebooster (1) is maintained at a substantially constant level; or b) thepower consumption of the booster (1) is maintained at a substantiallyconstant level; or c) the pressure difference between the outlet and theinlet of the booster (1) is maintained at a substantially constantlevel.
 9. The system of claim 8, wherein the flow booster (1) is a wetgas compressor of which at least one rotor comprises rotor blades havingnon-wettable sides and a leading edge having a larger roughness than thesides.
 10. The system of claim 9, wherein the leading edge of saidblades is covered-by a random array of grains having a grainsize of atleast about 50 microns, which grains are separated by distances whichare on average less than twice the grainsize.
 11. The system of claim 9,wherein the leading edge of said blades is provided with a series ofriblets which have a square or triangular cross-sectional shape, whichare substantially aligned with the direction of flow of the multi-phasefluid mixture and which have mutual spacings of at least 50 microns. 12.The system of claim 9, wherein the non-wettable sides of the rotorblades comprise a polytetrafluorethylene coating.
 13. A blade for use asa rotor or stator blade of a multi-phase flow booster (1) according toclaim 9, the blade having non-wettable sides and a leading edge having alarger roughness than the sides.
 14. The blade of claim 13, wherein theleading edge of said blade is covered by a random array of grains havinga grainsize of at least about 50 microns, which grains are separated bydistances which are on average less than twice the grainsize.
 15. Theblade of claim 13, wherein the leading edge of the blade is providedwith a series of riblets which have a square or triangularcross-sectional shape, which are substantially aligned with thedirection of flow of the multi-phase fluid mixture and which have mutualspacings of at least 50 microns.
 16. The blade of claim 13, wherein thenon-wettable sides of the blade comprise a polytetrafluorethylenecoating.